Many thanks to @erinn1910 for pointing this out on twitter.
From cropped photos from Daily Mail, you can see when Wick first encounters Zero the ring finger on his left hand is there and after the shot, it is wrapped with black tape giving the appearance that it is gone.
Maybe he has a boo-boo and they thought a black bandaid would be less intrusive and easier to fix in post-production or maybe not. If it is the later, the good news is that he shoots right handed. The bad news is he no longer has the wedding band Helen gave him and someone is responsible for that.
